The microwave resistance of superconductors and the status of high-temperature-superconductor materials technology in achieving low resistances is discussed. The resistance R of superconductors at microwave frequencies is equal to R=R(T)+R(res), where R (res) is a residual resistance due to such mechanisms as impurity scattering and R(T) is due to the thermal excitation of normal (as opposed to paired) electrons. Initial microwave measurements of ceramic yttrium barium copper oxide yielded R(res) values ranging from 0.1 to that of copper. Lower resistance values observed for thin films and single crystals are reported.
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